Low-energy Excited States Research Articles

The 130 – 750 GHz rotational spectrum of 2-cyanopyridine – Analysis of the ground vibrational state and the Coriolis-coupled dyad of its lowest-energy fundamental states

The millimeter-wave rotational spectrum of 2-cyanopyridine was collected from 130 to 750 GHz, and the ground and two lowest-energy excited vibrational states were analyzed. In total, over 20,000 rotational transitions were least-squares fit for the three vibrational states to partial-octic, distorted-rotor Hamiltonians with low error (σfit < 50 kHz). For the ground state, the many thousands of newly measured rotational transitions enabled substantial refinement of the rotational constants and determination of the centrifugal distortion constants. The rotational spectrum was collected at room temperature, permitting the observation of the two lowest-energy fundamental modes, ν30 (A″, 154 cm−1) and ν21 (A′, 175 cm−1), and determination of their spectroscopic constants. The two excited vibrational states are Coriolis coupled and require a two-state Hamiltonian. Eight Coriolis-coupling parameters (Ga,GaJ,GaK,GaJJ,Fbc,FbcJ,Gb,and GbJ) have been determined, as well as a precise energy difference of 26.524 312 6 (40) cm−1 between the vibrational states. A comparison of the ground-state spectroscopic constants, as well as the Coriolis coupling-related parameters of analogous dyads is presented for multiple cyanoarenes.

Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer.

Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d6-low-spin Co(III) photosensitizer [Co(4,4'-Br2bpy)3]3+ (where 4,4'-Br2bpy is 4,4'-dibromo-2,2'-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern-Volmer plots for each chromophore-quencher pair revealed a limiting value of Ered* ∼ 1.25 V vs Fc/Fc+ for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy)3]2+ (Ered* = 0.32 V vs Fc/Fc+) and [Ir(ppy)2(bpy)]+ (Ered* = 0.27 V vs Fc/Fc+). These results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

77 % Photothermal Conversion in Blatter-Type Diradicals: Photophysics and Photodynamic Applications.

Diradicals based on the Blatter units and connected by acetylene and alkene spacers have been prepared. All the molecules show sizably large diradical character and low energy singlet-triplet gaps. Their photo-physical properties concerning their lowest energy excited state have been studied in detail by steady-state and time-resolved absorption spectroscopy. We have fully identified the main optical absorption band and full absence of emission from the lowest energy excited state. A computational study has been also carried out that has helped to identify the presence of a conical intersection between the lowest energy excited state and the ground state which produces a highly efficient light-to-heat conversion of the absorbed radiation. Furthermore, an outstanding photo-thermal conversion 77.23 % has been confirmed, close to the highest in the diradicaloid field. For the first time, stable diradicals are applied to photo-thermal therapy of tumor cells with good stability and satisfactory performance at near-infrared region.

77 % Photothermal Conversion in Blatter‐Type Diradicals: Photophysics and Photodynamic Applications

AbstractDiradicals based on the Blatter units and connected by acetylene and alkene spacers have been prepared. All the molecules show sizably large diradical character and low energy singlet‐triplet gaps. Their photo‐physical properties concerning their lowest energy excited state have been studied in detail by steady‐state and time‐resolved absorption spectroscopy. We have fully identified the main optical absorption band and full absence of emission from the lowest energy excited state. A computational study has been also carried out that has helped to identify the presence of a conical intersection between the lowest energy excited state and the ground state which produces a highly efficient light‐to‐heat conversion of the absorbed radiation. Furthermore, an outstanding photo‐thermal conversion 77.23 % has been confirmed, close to the highest in the diradicaloid field. For the first time, stable diradicals are applied to photo‐thermal therapy of tumor cells with good stability and satisfactory performance at near‐infrared region.

Thermodynamic stability of cytosine tetramers mediated by partially reduced silver trimers: QM/MM free energy analysis

AbstractPartially reduced silver clusters growing in a polycytosine matrix could simultaneously possess stabilizing ability and remarkable optical features. The mechanisms of this phenomenon are not fully understood so far. We examined the thermodynamic stability of a cytosine–silver associate comprising four cytosines and three silver atoms that can be considered the smallest known luminescent silver marker stabilized on DNA. Quantum and molecular mechanical representations thermodynamic integration toward the dissociation pathway of the cytosine tetramer, time‐dependent density functional theory calculations of the lowest energy excited states and quantum theory of atoms in molecules analysis of bonding showed that the reduction of two ions within the silver trimer mediating cytosine tetramer leads to the appearance of the green‐emitting silver cluster with low stabilization ability of cytosines relative to a tetramer stabilized by two silver ions.

Weak Edgeworth expansion for the mean-field Bose gas

We consider the ground state and the low-energy excited states of a system of N identical bosons with interactions in the mean-field scaling regime. For the ground state, we derive a weak Edgeworth expansion for the fluctuations of bounded one-body operators, which yields corrections to a central limit theorem to any order in 1/sqrt{N}. For suitable excited states, we show that the limiting distribution is a polynomial times a normal distribution, and that higher-order corrections are given by an Edgeworth-type expansion.

Mechanoluminescent property of bis(phenylethynyl)platinum(II) complexes bearing substituted bipyridine and their application for developing rewritable data recording device

Two platinum(II) complexes, Pt(DiClBpy)(CCC6H5)2 (1) and Pt(DiBrBpy)(CCC6H5)2 (2) based on 4,4′-dichloro-2,2′-bipyridine (DiClbpy), 4,4′-dibromo-2,2′-bipyridine (DiBrbpy) and phenylacetylene ligands were synthesized and characterized. Both complexes display reversible and reproducible mechanochromic and mechanoluminescent properties with the luminescence red-shift exceeding 150 nm. Systematic studies on crystal structure, luminescence spectra, and PXRD demonstrate that the mechanochromic and mechanoluminescent properties of 1 and 2 are attributed to the formation/elimination of intermolecular Pt-Pt interactions during mechanical grinding/vapor absorption process, which lead to the transformation of the lowest energy excited states between MLCT and MMLCT transitions. In addition, based on such property, 1 and 2 were used as starting functional material to successfully develop the rewritable data recording device.

Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers.

Funneling excitation energy toward lower energy excited states is a key concept in photosynthesis, which is often realized with at most two chemically different types of pigment molecules. However, current synthetic approaches to establish energy funnels, or gradients, typically rely on Förster-type energy-transfer cascades along many chemically different molecules. Here, we demonstrate an elegant concept for a gradient in the excited-state energy landscape along micrometer-long supramolecular nanofibers based on the conjugated polymer poly(3-hexylthiophene), P3HT, as the single component. Precisely aligned P3HT nanofibers within a supramolecular superstructure are prepared by solution processing involving an efficient supramolecular nucleating agent. Employing hyperspectral imaging, we find that the lowest-energy exciton band edge continuously shifts to lower energies along the nanofibers' growth direction. We attribute this directed excited-state energy gradient to defect fractionation during nanofiber growth. Our concept provides guidelines for the design of supramolecular structures with an intrinsic energy gradient for nanophotonic applications.

Probing Charge-Transfer Processes in a Covalently Linked [Ge9 ]-Cluster Imine Dyad.

C60 donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge9 ] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge9 ] cluster linked to an extended π-electron system. [Ge9 {Si(TMS)3 }2 {CH3 C=N}-DAB(II)Dipp ]- (1- ) is formed upon the reaction of [Ge9 {Si(TMS)3 }2 ]2- with bromo-diazaborole DAB(II)Dipp -Br in CH3 CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1- yields the deep green, zwitterionic cluster [Ge9 {Si(TMS)3 }2 {CH3 C=N(H)}-DAB(II)Dipp ] (1-H) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.

Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II) Complexes.

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d3 chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t2)3 electron configuration involving the dπ levels (O point group symmetry labels). Pseudoctahedral 3d8 nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the dσ orbitals and (e)2 configurations. We have prepared the known nickel(II) complexes [Ni(terpy)2]2+, [Ni(phen)3]2+, and [Ni(ddpd)2]2+ as well as the novel complexes [Ni(dgpy)2]2+ and [Ni(tpe)2]2+ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.

The two-stepwise luminescent switching properties of triple-stimuli-responsive platinum(II) complexes bearing 4,4′-bis(2-phenylethynyl)-2,2′-bipyridine ligand

Three new square-planar platinum(II) complexes, Pt(PhC≡CbpyC≡CPh)(C≡CC6H4-X-2)2 [PhC≡CbpyC≡CPh = 4,4′-bis(2-phenylethynyl)-2,2′-bipyridine; X = F(1), Cl(2), and Br(3)] were synthesized and characterized. Upon exposure of the ground sample to volatile organic compound (VOC) vapors, a series of metastable structures of three complexes, including yellow solvent-free species 2 and 3, orange species 1, 2·ClCH2CH2Cl and 3·ClCH2CH2Cl, red species 1·1.5(CHCl3), 2·½(CH2Cl2), 2·½(ClCH2CH2Cl), 3·½(Toluene), and 3·½(ClCH2CH2Cl) were isolated. All three complexes exhibit vapor-, thermo-, and mechanical-grinding-triggered triple-stimuli-responsive luminescent switching properties. More interestingly, upon exposure to ClCH2CH2Cl vapor, the red species 2·½(CH2Cl2) or 3·½(Toluene) undergo an unusual time-dependent two-stepwise luminescent switching process and convert to the orange species 2·ClCH2CH2Cl or 3·ClCH2CH2Cl via intermediate 2·½(ClCH2CH2Cl) or 3·½(ClCH2CH2Cl). Furthermore, 1·1.5(CHCl3) also displays a two-stepwise thermoluminescent property and is transformed into orange species 1via intermediate 1·⅛(CHCl3). Furthermore, both 2 and 3 present the irreversible vapochromic and vapoluminescent properties as the yellow and orange species could not be converted to each other or transformed to the red species by absorbing the corresponding VOC vapors. Systematic studies revealed that the vapor-, thermo-, and mechanical-grinding-triggered triple-stimuli-responsive luminescence switching properties of 1–3 are due to the structural conversion and resultant changes of the lowest-energy excited states. The crystal structure, 1H NMR, luminescence spectra and PXRD measurements demonstrate that the two-stepwise luminescent switching behavior of complexes are associated with the stacking structure of species with same kind and different amount of solvent molecule. The irreversible vapoluminescent property of 2 and 3 are most likely due to the closed stacking of species which prevent the inserting of solvent molecules into the crystal lattice. In addition, complexes 1–3 have good application prospects in the visual identification of CHCl3, CH3CN and 1,2-dichloroethane vapors.

Non-Phenomenological Description of the Time-Resolved Emission in Solution with Quantum-Classical Vibronic Approaches-Application to Coumarin C153 in Methanol.

We report a joint experimental and theoretical work on the steady-state spectroscopy and time-resolved emission of the coumarin C153 dye in methanol. The lowest energy excited state of this molecule is characterized by an intramolecular charge transfer thus leading to remarkable shifts of the time-resolved emission spectra, dictated by the methanol reorganization dynamics. We selected this system as a prototypical test case for the first application of a novel computational protocol aimed at the prediction of transient emission spectral shapes, including both vibronic and solvent effects, without applying any phenomenological broadening. It combines a recently developed quantum-classical approach, the adiabatic molecular dynamics generalized vertical Hessian method (Ad-MD|gVH), with nonequilibrium molecular dynamics simulations. For the steady-state spectra we show that the Ad-MD|gVH approach is able to reproduce quite accurately the spectral shapes and the Stokes shift, while a ∼0.15 eV error is found on the prediction of the solvent shift going from gas phase to methanol. The spectral shape of the time-resolved emission signals is, overall, well reproduced, although the simulated spectra are slightly too broad and asymmetric at low energies with respect to experiments. As far as the spectral shift is concerned, the calculated spectra from 4 ps to 100 ps are in excellent agreement with experiments, correctly predicting the end of the solvent reorganization after about 20 ps. On the other hand, before 4 ps solvent dynamics is predicted to be too fast in the simulations and, in the sub-ps timescale, the uncertainty due to the experimental time resolution (300 fs) makes the comparison less straightforward. Finally, analysis of the reorganization of the first solvation shell surrounding the excited solute, based on atomic radial distribution functions and orientational correlations, indicates a fast solvent response (≈100 fs) characterized by the strengthening of the carbonyl-methanol hydrogen bond interactions, followed by the solvent reorientation, occurring on the ps timescale, to maximize local dipolar interactions.

Synthesis and functionalization of electron-deficient perfluoroalkyl porphyrin building blocks for supermolecular systems

Synthetic strategies for electron-deficient meso-perfluoroalkylporphyrins bearing diverse functional groups are described. Scalable and efficient syntheses for 5-triisopropylsilylethynyl-10,15,20-tris(heptafluoropropyl)porphyrin and 5-triisopropylsilylethynyl-10,20-bis(heptafluoropropyl)porphyrin that equip meso-ethynyl functional groups via the bilane route have been established, along with a refined route to [5,15-bis(heptafluoropropyl)porphinato]zinc(II). meso-Position halogenation of [5,15-bis(heptafluoropropyl)porphinato]zinc(II) was achieved by selective meso-nitration and subsequent reduction, diazonium salt formation, and iodination reactions. Computational data describe the low energy excited states of these chromophores and the electronic structural factors that control reactivity of these meso-perfluoroalkyl substituted porphyrin complexes. meso-Functionalized [5-triisopropylsilylethynyl-10,20-bis(heptafluoropropyl)porphinato]zinc(II) and [5-iodo-10,20-bis(heptafluoropropyl)porphinato]zinc(II) building blocks lay the foundation for the construction of highly conjugated multiporphyrin arrays that feature electronic structural properties important for the development of n-type materials and high potential photooxidants.

Control and Enhancement of Single-Molecule Electroluminescence through Strong Light–Matter Coupling

The energetic positions of molecular electronic states at molecule/electrode interfaces are crucial factors for determining the transport and optoelectronic properties of molecular junctions. Strong light-matter coupling offers a potential for manipulating these factors, enabling a boost in the efficiency and versatility of these junctions. Here, we investigate electroluminescence from single-molecule junctions in which the molecule is strongly coupled with the vacuum electromagnetic field in a plasmonic nanocavity. We demonstrate an improvement in the electroluminescence efficiency by employing the strong light-matter coupling in conjunction with the characteristic feature of single-molecule junctions to selectively control the formation of the lowest-energy excited state. The mechanism of efficiency improvement is discussed based on the energetic position and composition of the formed polaritonic states. Our findings indicate the possibility to manipulate optoelectronic conversion in molecular junctions by strong light-matter coupling and contribute to providing design principles for developing efficient molecular optoelectronic devices.

The Resonance Raman Spectrum of Cytosine in Water: Analysis of the Effect of Specific Solute-Solvent Interactions and Non-Adiabatic Couplings.

In this contribution, we report a computational study of the vibrational Resonance Raman (vRR) spectra of cytosine in water, on the grounds of potential energy surfaces (PES) computed by time-dependent density functional theory (TD-DFT) and CAM-B3LYP and PBE0 functionals. Cytosine is interesting because it is characterized by several close-lying and coupled electronic states, challenging the approach commonly used to compute the vRR for systems where the excitation frequency is in quasi-resonance with a single state. We adopt two recently developed time-dependent approaches, based either on quantum dynamical numerical propagations of vibronic wavepackets on coupled PES or on analytical correlation functions for cases in which inter-state couplings were neglected. In this way, we compute the vRR spectra, considering the quasi-resonance with the eight lowest-energy excited states, disentangling the role of their inter-state couplings from the mere interference of their different contributions to the transition polarizability. We show that these effects are only moderate in the excitation energy range explored by experiments, where the spectral patterns can be rationalized from the simple analysis of displacements of the equilibrium positions along the different states. Conversely, at higher energies, interference and inter-state couplings play a major role, and the adoption of a fully non-adiabatic approach is strongly recommended. We also investigate the effect of specific solute-solvent interactions on the vRR spectra, by considering a cluster of cytosine, hydrogen-bonded by six water molecules, and embedded in a polarizable continuum. We show that their inclusion remarkably improves the agreement with the experiments, mainly altering the composition of the normal modes, in terms of internal valence coordinates. We also document cases, mostly for low-frequency modes, in which a cluster model is not sufficient, and more elaborate mixed quantum classical approaches, in explicit solvent models, need to be applied.

Localization of nuclear wave functions of lithium in [Li+@C60]PF6-: molecular insights into two-site disorder-order transition.

Lithium endohedral fullerene, Li+@C60, is a porous system ideal for studying the quantized translational motion of the Li+ nucleus under subnanoscale confinement. The quantized nuclear motion strongly depends on the anharmonic and polarizable adsorbent potential within the C60 cage, which can be perturbed by cage distortion and/or exterior ions. In our recent paper, H. Ando and Y. Nakao, Phys. Chem. Chem. Phys., 2021, 23, 9785-9803, we focused on a [Li+@C60]PF6- salt and theoretically investigated how the Li+ ion in each C60 cage is simultaneously localized at two equivalent disordered sites in 24 K < T ≪ 100 K. At 24 K, the salt exhibits a disorder-order transition, whereby every Li+ ion becomes mostly localized at one of the two disordered sites below that temperature. Herein we discuss the origin of this transition with special attention to the local structural distortion and intermolecular interactions. Using the Fourier grid Hamiltonian method and a model function that fits a post-Hartree-Fock potential energy surface, we obtained hundreds of low-energy nuclear wave functions of Li+ confined within the cage. The weak distortions of the C60 cage and the PF6- coordination sphere below 24 K and concurrent inversion-symmetry breaking affect intermolecular interactions, thus making the wave functions of the nuclear ground state and several low-energy excited states localized around the experimental high-occupancy disordered site. This demonstrates that the distortions correlate closely with the two-site disorder-order transition. Finally, we reveal that two absorption peaks in the terahertz frequency range show substantial blueshifts upon cooling below 24 K, which serve as fingerprints of the transition.

Effects of Proaromaticity on Excited-State Lifetimes and Charge Separation in Near-Infrared Sensitizer Dyes in Solution and on TiO2

The influence of proaromaticity on the excited state dynamics of near-infrared (NIR)-absorbing sensitizer dyes is explored using a combination of ultrafast transient absorption spectroscopy (TAS) and computational chemistry. The addition of a proaromatic π-bridge was found to stabilize the excited state and lead to lower excitation energies in solution and longer excited-state lifetimes, contrary to what is expected by the energy gap law. When studied under standard device conditions on TiO2, it was found that the dye structure plays a significant role in determining excited-state dynamics. Computational chemistry results confirm the proaromatic nature of the dyes through both bond length analyses and nucleus-independent chemical shift (NICS) calculations. Through incorporation of excited-state aromaticity, a 10× increase in excited-state lifetime was observed for dyes with a near 0.5 V lower energy excited state.

Micellization Turned on Dual Fluorescence and Room Temperature Phosphorescence by Pseudo-ESIPT in Thiadiazole Derivatives

Some luminophores cannot fluoresce from dilute solution, especially those that are highly flexible. By aggregation, the rigidity of these molecules increases allowing for photoluminescence. However, the enhanced rigidity and the intermolecular interactions can give rise to several high-energy excited states, and since they can fluoresce only from the lowest energy excited state, known as the Kasha rule, the energy surplus is lost to the surroundings. In the pursuit of developing efficient light harvesting strategies from the higher energy excited states, it is necessary to look upon the effects not only at the level of nanometric molecules themselves but also of their supramolecular assemblies. Herein, we pinpoint the impact of poor conjugated moieties over the chromophore units, as well as the optical phenomena emerging at the supramolecular level. The molecular design consists of the S-alkylation of 5-amino-1,3,4-thiadiazole-2-thiol with permethylated silicone or hydrocarbons. The synthesized compounds display excited state intermolecular proton transfer (pseudo-ESIPT), blue and green fluorescence, and near-infrared phosphorescence at room temperature.

Laser-driven ionization mechanisms of aluminum for single particle aerosol mass spectrometry

Single particle aerosol mass spectrometry (SPAMS), an analytical technique for measuring the size and composition of individual micron-scale particles, is capable of analyzing atmospheric pollutants and bioaerosols much more efficiently and with more detail than conventional methods which require the collection of particles onto filters for analysis in the laboratory. Despite SPAMS’ demonstrated capabilities, the primary mechanisms of ionization are not fully understood, which creates challenges in optimizing and interpreting SPAMS signals. In this paper, we present a well-stirred reactor model for the reactions involved with the laser-induced vaporization and ionization of an individual particle. The SPAMS conditions modeled in this paper include a 248 nm laser which is pulsed for 8 ns to vaporize and ionize each particle in vacuum. The ionization of 1 μm, spherical Al particles was studied by approximating them with a 0-dimensional plasma chemistry model. The primary mechanism of absorption of the 248 nm photons was pressure-broadened direct photoexcitation to Al(y2D). Atoms in this highly excited state then undergo superelastic collisions with electrons, heating the electrons and populating the lower energy excited states. We found that the primary ionization mechanism is electron impact ionization of various excited state Al atoms, especially Al(y2D). Because the gas expands rapidly into vacuum, its temperature decreases rapidly. The rate of three-body recombination (e− + e− + Al+ → Al + e−) increases at low temperature, and most of the electrons and ions produced recombine within several μs of the laser pulse. The importance of the direct photoexcitation indicates that the relative peak heights of different elements in SPAMS mass spectra may be sensitive to the available photoexcitation transitions. The effects of laser intensity, particle diameter, and expansion dynamics are also discussed.

Delayed fluorescence from inverted singlet and triplet excited states

Hund’s multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration1. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund’s rule and has a negative singlet–triplet energy gap of −11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers.